How to Make Floors Warmer Over a Crawl Space in Winter

By: Danny Lipford

Can I install a plastic sheet over the existing insulation under the crawl space of my house without causing mold to the insulation? The floor inside the house seems cold in the winter, and I would like to try to keep the air in the crawl space from affecting the heating of the living space. -Glen

Hi Glen,

I would advise against it, since applying plastic to the bottom of the floor joints in the crawl space under your house can trap moisture from the air in the insulation which can result in mold and rot.

A better approach would be to cover the ground of the crawl space with plastic to reduce the moisture, and reduce the ventilation in your crawl space during the winter. This will lower the humidity while raising the temperature of the air under your home during the winter, which in turn will make the floors above feel warmer.

While it’s important to have ventilation in your crawl space, if it’s open to the outside, you should enclose the foundation, and install vents that can be closed during the winter. Foundation vents are available with a temperature sensitive metal spring that causes the vent to close automatically when the temperature drops. If your crawl space has open latticework around it, attach landscape fabric to the back of the lattice to reduce the amount of air passing through.

7 Comments on “How to Make Floors Warmer Over a Crawl Space in Winter”

From a fellow Mobilian;
I have a 20 inch crawl space in my approx. 1400 ft2 house in Mobile AL.
I have enclosed the perimeter with landscape cloth and lattice. Should I open each side for ventilation?
Will this cause mold and wood rot?

Is this practice acceptable for Mobile weather?? Any advice will be appreciated?

We have an addition on the back of our home that is raised roughly 4-5 feet from the ground. The floor on the addition gets really cold in the winter. I’d like to completely enclose it and put a foundation vent in, like mentioned. However, our air conditioning unit is under this space and the exhaust from our washer/dryer is under it too. Can I still completely close off the space and use a foundation vent, or should I worry about pressure, moisture, or consider anything else? Thank you.

I live in a historical section in MA. My house was built in 1744. Its beams, siles. and flooring are in excellent shape. Looking at the house from the end you can see the porches are a mirrored image to each opposite side of the house.The problem is they only go half the length of the house. Leaving 2 crawl spaces that you cant get into, and the floors are extremely cold. The crawl space is very dry with all suspected openings being closed to eliminate draft. Do you have some method you would reconise that might hep me? Thanks

Yesterday I had insulation put inbetween joists under my manufactured home. They said the insulation fit tight and would not fall. They didn’t use anything to hold it up….like wire supports. I am concerned it will eventually fall onto the polythylene barrier. They said the white sheet of polythylene barrier will hold it up. It is an enclosed crawl space. Please respond. I am afraid they will not see what I am concerned about. I Tryed discussing it with the guy running the job.

I have been told it is a good idea to encapsulate my crawl space completely. It would have a perimeter gravity drain for any water getting under the house and a dehumidifier. I understand this would eliminate the need to insulate between the joists and I would not need to wrap furnace duct work. Please let me know if this is a good solution.
Thank you.

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How to Make Floors Warmer Over a Crawl Space in Winter

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